Conifers can be traced back 350 million years (to the Carboniferous). While swamps were dominated by horsetails and seed ferns, drier uplands were host to conifer trees. Early conifers were probably only 2-3 m (about 10 feet) in height. Leaves of these early conifers were needle-like but their cones were not as compact as modern cones. By the Permian, conifers had diversified into many forms, including large, woody trees much like modern conifers. At the end of the Permian, an enormous extinction event caused a major change in land ecosystems. Many of the primitive conifers went extinct, but survivors gradually spread during the Triassic until they became dominant forest trees. The early and middle Mesozoic (about 250-130 million years ago) became the age of conifers. Like modern forms, Mesozoic conifers were large, woody plants that made seeds. With the exception of one group that went extinct (the Cheirolepidiaceae), families of Mesozoic conifers are still on Earth today.

The great biogeographer Wladimir Peter Koppen once said that plants are crystalized visible climate. He had studied the distribution of modern plants, but there is no reason to believe that ancient plants were not equally sensitive to climate. Indicators of paleoclimate, such as rainfall and surface temperature, can be found in the chemistry of fossil plants and the rocks that surround them. The form of the fossils themselves can also reveal a great deal about climate. For example, plants have tiny openings on their leaves (stomata) through which they absorb CO2 and release oxygen. More stomata occur in low CO2 atmospheres, and fewer in high CO2 environments. Some woody plants have growth rings, showing the alternation of favorable and unfavorable conditions. Leaf shapes can also act as thermometers. Leaves with serrated edges (toothed margins) are more common in cooler climates, whereas smooth-edged leaves dominate in warmer climates. By studying modern forests, and applying the findings to extinct plant communities, past climate conditions can be inferred. Changes in fossil plant assemblages mirror changes in global climate over time.